An example of a technique of optically detecting the discharging state of ink from a printhead is disclosed in, e.g., U.S. Pat. Nos. 5,276,467, 5,350,929, and 5,376,958.
A technique of detecting, at high precision, very small ink droplets discharged from each nozzle of a printhead is disclosed in, e.g., U.S. Pat. No. 5,434,430. In these conventional techniques, an analog signal to be generated by detecting ink discharged from each nozzle is compared with a reference value to determine discharge/non-discharge of ink from the nozzle.
For example, according to U.S. Pat. No. 6,517,183, the peak value (peak to peak) of an analog signal to be obtained by detecting discharged ink is evaluated to determine discharge or non-discharge. A technique of changing a determination threshold in accordance with the type of printhead is disclosed in, e.g., U.S. Pat. Nos. 6,056,386, and 6,419,341.
According to these conventional techniques, the difference between detected results is reduced by changing the determination threshold in accordance with the characteristic difference in droplet size, ink color, or a type of printhead, or performing ink discharge for compensating for such difference. Each conventional method essentially employs a detection method having a threshold fixed for each condition.
The principal object of these conventional techniques is to determine discharge/non-discharge of ink, e.g., whether ink (droplet) has been discharged from each nozzle or not. The precision of determining whether appropriate discharge has been done for each nozzle of a printhead is low. In other words, even in the use of a technique of correcting the detection error due to droplet size or ink color, the conventional techniques can attain a precision only enough to prevent misjudgment of discharge/non-discharge from nozzles by a detection mechanism or circuit in consideration of variations in ink-jet printer apparatus, printhead (ink-jet head), ink, environment, or the like.
In recent years, the resolution of an ink-jet printer has increased more and more, and higher-quality image printing is required. In this situation, determination of the ink discharge from each nozzle is not merely determination of discharge/non-discharge. It is necessary to determine whether proper ink discharge related to the quality of a printed image has been done, and feed back the determination result to an ink-jet printer apparatus.